1. Field of the Invention
This invention relates to a developing device in electrophotography, and more specifically, to a device for developing a latent electrostatic image by using a two-component developer composed of a ferrite carrier and an electrically chargeable toner.
2. Description of the Prior Art
In an electrophotographic device using a two-component magnetic developer, a chargeable toner and a magnetic carrier are mixed to form a two-component composition. The composition is supplied to a development sleeve having a magnet therein to form a magnetic brush composed of the composition. An electrophotographic plate bearing a latent electrostatic image is brought into rubbing contact with the magnetic brush to form a charged toner image on the photographic plate. By rubbing with the magnetic carrier, the toner is charged to a polarity opposite to the polarity of the charged electrostatic latent image on the photographic plate. Consequently, the charged toner particles on the magnetic brush adhere to the latent electrostatic image by the Coulomb's force to develop the latent electrostatic image to a visible image. On the other hand, the magnetic carrier is attracted to the magnet within the sleeve, and the polarity of its charge is the same as the polarity of the charged latent electrostatic image. Hence, the magnetic carrier remains on the sleeve.
Generally, an iron powder carrier is widely used as the magnetic carrier, but the iron powder carrier still has many defects. A two-component developer containing the iron powder carrier has the defect that the rising of the development sensitivity curve (a curve showing the relation of the density of an image to the potential between the latent electrostatic image and the development sleeve) is steep, and the image has an inferior gradation and a poor reproducibility of a halftone. Furthermore, the developer containing the iron powder carrier sometimes forms a hard magnetic brush and may possibly injure the photosensitive layer. Moreover, in copying a solid portion, brush marks, which are many rows of short and slender white lines extending in the rubbing direction of the brush, are seen to form. Another disadvantage is that the iron powder carrier is sensitive to humidity, and its development characteristics may change under the influences of humidity. Or rust tends to occur in the iron powder carrier itself.
To remedy these defects, the use of ferrite, particularly soft ferrite, as the magnetic carrier has recently been proposed. Since, however, the ferrite carrier is very different from the iron powder carrier in particle characteristics and magnetic properties, the developing conditions used with the iron powder carrier cannot be applied directly. For example, the iron powder carrier consists of irregularly-shaped particles, whereas the ferrite carrier is composed of nearly spherical particles. Furthermore, the ferrite carrier has a lower permeability than the iron powder carrier. For this reason, a two-component magnetic developer containing the ferrite carrier forms a magnetic brush having a smaller brush length than the developer containing the iron powder carrier. Thus, a toner image of good quality cannot be obtained unless the brush length is considerably shortened. When the brush length of the two-component magnetic developer containing the ferrite carrier is so shortened, the rotation of the development sleeve requires an excessive torque which will cause deflection or vibration in the development device so that development cannot be performed well. Or the excessive torque also causes deflection to the optical unit.